JNI tips

JNI is the Java Native Interface. It defines a way for the bytecode that Android compiles from
managed code (written in the Java or Kotlin programming languages) to interact with native code
(written in C/C++). JNI is vendor-neutral, has support for loading code from dynamic shared
libraries, and while cumbersome at times is reasonably efficient.

Note: Because Android compiles Kotlin to ART-friendly bytecode in
a similar manner as the Java programming language, you can apply the guidance on this page to both
the Kotlin and Java programming languages in terms of JNI architecture and its associated costs.
To learn more, see
Kotlin and Android.

If you're not already familiar with it, read through the
Java Native Interface Specification
to get a sense for how JNI works and what features are available. Some
aspects of the interface aren't immediately obvious on
first reading, so you may find the next few sections handy.

To browse global JNI references and see where global JNI references are created and deleted, use
the JNI heap view in the Memory Profiler
in Android Studio 3.2 and higher.

General tips

Try to minimize the footprint of your JNI layer. There are several dimensions to consider here.
Your JNI solution should try to follow these guidelines (listed below by order of importance,
beginning with the most important):

Minimize marshalling of resources across the JNI layer. Marshalling across
the JNI layer has non-trivial costs. Try to design an interface that minimizes the amount of
data you need to marshall and the frequency with which you must marshall data.

Avoid asynchronous communication between code written in a managed programming
language and code written in C++ when possible.
This will keep your JNI interface easier to maintain. You can typically simplify asynchronous
UI updates by keeping the async update in the same language as the UI. For example, instead of
invoking a C++ function from the UI thread in the Java code via JNI, it's better
to do a callback between two threads in the Java programming language, with one of them
making a blocking C++ call and then notifying the UI thread when the blocking call is
complete.

Minimize the number of threads that need to touch or be touched by JNI.
If you do need to utilize thread pools in both the Java and C++ languages, try to keep JNI
communication between the pool owners rather than between individual worker threads.

Keep your interface code in a low number of easily identified C++ and Java source
locations to facilitate future refactors. Consider using a JNI auto-generation
library as appropriate.

JavaVM and JNIEnv

JNI defines two key data structures, "JavaVM" and "JNIEnv". Both of these are essentially
pointers to pointers to function tables. (In the C++ version, they're classes with a
pointer to a function table and a member function for each JNI function that indirects through
the table.) The JavaVM provides the "invocation interface" functions,
which allow you to create and destroy a JavaVM. In theory you can have multiple JavaVMs per process,
but Android only allows one.

The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as
the first argument.

The JNIEnv is used for thread-local storage. For this reason, you cannot share a JNIEnv between threads.
If a piece of code has no other way to get its JNIEnv, you should share
the JavaVM, and use GetEnv to discover the thread's JNIEnv. (Assuming it has one; see AttachCurrentThread below.)

The C declarations of JNIEnv and JavaVM are different from the C++
declarations. The "jni.h" include file provides different typedefs
depending on whether it's included into C or C++. For this reason it's a bad idea to
include JNIEnv arguments in header files included by both languages. (Put another way: if your
header file requires #ifdef __cplusplus, you may have to do some extra work if anything in
that header refers to JNIEnv.)

Threads

All threads are Linux threads, scheduled by the kernel. They're usually
started from managed code (using Thread.start),
but they can also be created elsewhere and then attached to the JavaVM. For
example, a thread started with pthread_create can be attached
with the JNI AttachCurrentThread or
AttachCurrentThreadAsDaemon functions. Until a thread is
attached, it has no JNIEnv, and cannot make JNI calls.

Attaching a natively-created thread causes a java.lang.Thread
object to be constructed and added to the "main" ThreadGroup,
making it visible to the debugger. Calling AttachCurrentThread
on an already-attached thread is a no-op.

Android does not suspend threads executing native code. If
garbage collection is in progress, or the debugger has issued a suspend
request, Android will pause the thread the next time it makes a JNI call.

Threads attached through JNI must call
DetachCurrentThread before they exit.
If coding this directly is awkward, in Android 2.0 (Eclair) and higher you
can use pthread_key_create to define a destructor
function that will be called before the thread exits, and
call DetachCurrentThread from there. (Use that
key with pthread_setspecific to store the JNIEnv in
thread-local-storage; that way it'll be passed into your destructor as
the argument.)

jclass, jmethodID, and jfieldID

If you want to access an object's field from native code, you would do the following:

Get the class object reference for the class with FindClass

Get the field ID for the field with GetFieldID

Get the contents of the field with something appropriate, such as
GetIntField

Similarly, to call a method, you'd first get a class object reference and then a method ID. The IDs are often just
pointers to internal runtime data structures. Looking them up may require several string
comparisons, but once you have them the actual call to get the field or invoke the method
is very quick.

If performance is important, it's useful to look the values up once and cache the results
in your native code. Because there is a limit of one JavaVM per process, it's reasonable
to store this data in a static local structure.

The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded. Classes
are only unloaded if all classes associated with a ClassLoader can be garbage collected,
which is rare but will not be impossible in Android. Note however that
the jclass
is a class reference and must be protected with a call
to NewGlobalRef (see the next section).

If you would like to cache the IDs when a class is loaded, and automatically re-cache them
if the class is ever unloaded and reloaded, the correct way to initialize
the IDs is to add a piece of code that looks like this to the appropriate class:

Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code
will be executed once, when the class is initialized. If the class is ever unloaded and
then reloaded, it will be executed again.

Local and global references

Every argument passed to a native method, and almost every object returned
by a JNI function is a "local reference". This means that it's valid for the
duration of the current native method in the current thread.
Even if the object itself continues to live on after the native method
returns, the reference is not valid.

This applies to all sub-classes of jobject, including
jclass, jstring, and jarray.
(The runtime will warn you about most reference mis-uses when extended JNI
checks are enabled.)

The only way to get non-local references is via the functions
NewGlobalRef and NewWeakGlobalRef.

If you want to hold on to a reference for a longer period, you must use
a "global" reference. The NewGlobalRef function takes the
local reference as an argument and returns a global one.
The global reference is guaranteed to be valid until you call
DeleteGlobalRef.

This pattern is commonly used when caching a jclass returned
from FindClass, e.g.:

All JNI methods accept both local and global references as arguments.
It's possible for references to the same object to have different values.
For example, the return values from consecutive calls to
NewGlobalRef on the same object may be different.
To see if two references refer to the same object,
you must use the IsSameObject function. Never compare
references with == in native code.

One consequence of this is that you
must not assume object references are constant or unique
in native code. The 32-bit value representing an object may be different
from one invocation of a method to the next, and it's possible that two
different objects could have the same 32-bit value on consecutive calls. Do
not use jobject values as keys.

Programmers are required to "not excessively allocate" local references. In practical terms this means
that if you're creating large numbers of local references, perhaps while running through an array of
objects, you should free them manually with
DeleteLocalRef instead of letting JNI do it for you. The
implementation is only required to reserve slots for
16 local references, so if you need more than that you should either delete as you go or use
EnsureLocalCapacity/PushLocalFrame to reserve more.

Note that jfieldIDs and jmethodIDs are opaque
types, not object references, and should not be passed to
NewGlobalRef. The raw data
pointers returned by functions like GetStringUTFChars
and GetByteArrayElements are also not objects. (They may be passed
between threads, and are valid until the matching Release call.)

One unusual case deserves separate mention. If you attach a native
thread with AttachCurrentThread, the code you are running will
never automatically free local references until the thread detaches. Any local
references you create will have to be deleted manually. In general, any native
code that creates local references in a loop probably needs to do some manual
deletion.

Be careful using global references. Global references can be unavoidable, but they are difficult
to debug and can cause difficult-to-diagnose memory (mis)behaviors. All else being equal, a
solution with fewer global references is probably better.

UTF-8 and UTF-16 strings

The Java programming language uses UTF-16. For convenience, JNI provides methods that work with
Modified UTF-8 as well. The
modified encoding is useful for C code because it encodes \u0000 as 0xc0 0x80 instead of 0x00.
The nice thing about this is that you can count on having C-style zero-terminated strings,
suitable for use with standard libc string functions. The down side is that you cannot pass
arbitrary UTF-8 data to JNI and expect it to work correctly.

If possible, it's usually faster to operate with UTF-16 strings. Android
currently does not require a copy in GetStringChars, whereas
GetStringUTFChars requires an allocation and a conversion to
UTF-8. Note that
UTF-16 strings are not zero-terminated, and \u0000 is allowed,
so you need to hang on to the string length as well as
the jchar pointer.

Don't forget to Release the strings you Get. The
string functions return jchar* or jbyte*, which
are C-style pointers to primitive data rather than local references. They
are guaranteed valid until Release is called, which means they are not
released when the native method returns.

Data passed to NewStringUTF must be in Modified UTF-8 format. A
common mistake is reading character data from a file or network stream
and handing it to NewStringUTF without filtering it.
Unless you know the data is valid MUTF-8 (or 7-bit ASCII, which is a compatible subset),
you need to strip out invalid characters or convert them to proper Modified UTF-8 form.
If you don't, the UTF-16 conversion is likely to provide unexpected results.
CheckJNI—which is on by default for emulators—scans strings
and aborts the VM if it receives invalid input.

Primitive arrays

JNI provides functions for accessing the contents of array objects.
While arrays of objects must be accessed one entry at a time, arrays of
primitives can be read and written directly as if they were declared in C.

To make the interface as efficient as possible without constraining
the VM implementation, the Get<PrimitiveType>ArrayElements
family of calls allows the runtime to either return a pointer to the actual elements, or
allocate some memory and make a copy. Either way, the raw pointer returned
is guaranteed to be valid until the corresponding Release call
is issued (which implies that, if the data wasn't copied, the array object
will be pinned down and can't be relocated as part of compacting the heap).
You must Release every array you Get. Also, if the Get
call fails, you must ensure that your code doesn't try to Release a NULL
pointer later.

You can determine whether or not the data was copied by passing in a
non-NULL pointer for the isCopy argument. This is rarely
useful.

The Release call takes a mode argument that can
have one of three values. The actions performed by the runtime depend upon
whether it returned a pointer to the actual data or a copy of it:

0

Actual: the array object is un-pinned.

Copy: data is copied back. The buffer with the copy is freed.

JNI_COMMIT

Actual: does nothing.

Copy: data is copied back. The buffer with the copy
is not freed.

JNI_ABORT

Actual: the array object is un-pinned. Earlier
writes are not aborted.

Copy: the buffer with the copy is freed; any changes to it are lost.

One reason for checking the isCopy flag is to know if
you need to call Release with JNI_COMMIT
after making changes to an array — if you're alternating between making
changes and executing code that uses the contents of the array, you may be
able to
skip the no-op commit. Another possible reason for checking the flag is for
efficient handling of JNI_ABORT. For example, you might want
to get an array, modify it in place, pass pieces to other functions, and
then discard the changes. If you know that JNI is making a new copy for
you, there's no need to create another "editable" copy. If JNI is passing
you the original, then you do need to make your own copy.

It is a common mistake (repeated in example code) to assume that you can skip the Release call if
*isCopy is false. This is not the case. If no copy buffer was
allocated, then the original memory must be pinned down and can't be moved by
the garbage collector.

Also note that the JNI_COMMIT flag does not release the array,
and you will need to call Release again with a different flag
eventually.

Region calls

There is an alternative to calls like Get<Type>ArrayElements
and GetStringChars that may be very helpful when all you want
to do is copy data in or out. Consider the following:

This grabs the array, copies the first len byte
elements out of it, and then releases the array. Depending upon the
implementation, the Get call will either pin or copy the array
contents.
The code copies the data (for perhaps a second time), then calls Release; in this case
JNI_ABORT ensures there's no chance of a third copy.

One can accomplish the same thing more simply:

env->GetByteArrayRegion(array, 0, len, buffer);

This has several advantages:

Requires one JNI call instead of 2, reducing overhead.

Doesn't require pinning or extra data copies.

Reduces the risk of programmer error — no risk of forgetting
to call Release after something fails.

Similarly, you can use the Set<Type>ArrayRegion call
to copy data into an array, and GetStringRegion or
GetStringUTFRegion to copy characters out of a
String.

Exceptions

You must not call most JNI functions while an exception is pending.
Your code is expected to notice the exception (via the function's return value,
ExceptionCheck, or ExceptionOccurred) and return,
or clear the exception and handle it.

The only JNI functions that you are allowed to call while an exception is
pending are:

DeleteGlobalRef

DeleteLocalRef

DeleteWeakGlobalRef

ExceptionCheck

ExceptionClear

ExceptionDescribe

ExceptionOccurred

MonitorExit

PopLocalFrame

PushLocalFrame

Release<PrimitiveType>ArrayElements

ReleasePrimitiveArrayCritical

ReleaseStringChars

ReleaseStringCritical

ReleaseStringUTFChars

Many JNI calls can throw an exception, but often provide a simpler way
of checking for failure. For example, if NewString returns
a non-NULL value, you don't need to check for an exception. However, if
you call a method (using a function like CallObjectMethod),
you must always check for an exception, because the return value is not
going to be valid if an exception was thrown.

Note that exceptions thrown by interpreted code do not unwind native stack
frames, and Android does not yet support C++ exceptions.
The JNI Throw and ThrowNew instructions just
set an exception pointer in the current thread. Upon returning to managed
from native code, the exception will be noted and handled appropriately.

Native code can "catch" an exception by calling ExceptionCheck or
ExceptionOccurred, and clear it with
ExceptionClear. As usual,
discarding exceptions without handling them can lead to problems.

There are no built-in functions for manipulating the Throwable object
itself, so if you want to (say) get the exception string you will need to
find the Throwable class, look up the method ID for
getMessage "()Ljava/lang/String;", invoke it, and if the result
is non-NULL use GetStringUTFChars to get something you can
hand to printf(3) or equivalent.

Extended checking

JNI does very little error checking. Errors usually result in a crash. Android also offers a mode called CheckJNI, where the JavaVM and JNIEnv function table pointers are switched to tables of functions that perform an extended series of checks before calling the standard implementation.

The additional checks include:

Arrays: attempting to allocate a negative-sized array.

Bad pointers: passing a bad jarray/jclass/jobject/jstring to a JNI call, or passing a NULL pointer to a JNI call with a non-nullable argument.

Class names: passing anything but the “java/lang/String” style of class name to a JNI call.

Critical calls: making a JNI call between a “critical” get and its corresponding release.

Direct ByteBuffers: passing bad arguments to NewDirectByteBuffer.

Exceptions: making a JNI call while there’s an exception pending.

JNIEnv*s: using a JNIEnv* from the wrong thread.

jfieldIDs: using a NULL jfieldID, or using a jfieldID to set a field to a value of the wrong type (trying to assign a StringBuilder to a String field, say), or using a jfieldID for a static field to set an instance field or vice versa, or using a jfieldID from one class with instances of another class.

In either of these cases, you’ll see something like this in your logcat output when the runtime starts:

D AndroidRuntime: CheckJNI is ON

If you have a regular device, you can use the following command:

adb shell setprop debug.checkjni 1

This won’t affect already-running apps, but any app launched from that point on will have CheckJNI enabled. (Change the property to any other value or simply rebooting will disable CheckJNI again.) In this case, you’ll see something like this in your logcat output the next time an app starts:

D Late-enabling CheckJNI

You can also set the android:debuggable attribute in your application's manifest to
turn on CheckJNI just for your app. Note that the Android build tools will do this automatically for
certain build types.

Native libraries

In practice, older versions of Android had bugs in PackageManager that caused installation and
update of native libraries to be unreliable. The ReLinker
project offers workarounds for this and other native library loading problems.

Call System.loadLibrary (or ReLinker.loadLibrary) from a static class
initializer. The argument is the "undecorated" library name,
so to load libfubar.so you would pass in "fubar".

There are two ways that the runtime can find your native methods. You can either explicitly
register them with RegisterNatives, or you can let the runtime look them up dynamically
with dlsym. The advantages of RegisterNatives are that you get up-front
checking that the symbols exist, plus you can have smaller and faster shared libraries by not
exporting anything but JNI_OnLoad. The advantage of letting the runtime discover your
functions is that it's slightly less code to write.

In your JNI_OnLoad, register all of your native methods using RegisterNatives.

Build with -fvisibility=hidden so that only your JNI_OnLoad
is exported from your library. This produces faster and smaller code, and avoids potential
collisions with other libraries loaded into your app (but it creates less useful stack traces
if you app crashes in native code).

The static initializer should look like this:

Kotlin

companion object {
init {
System.loadLibrary("fubar")
}
}

Java

static {
System.loadLibrary("fubar");
}

The JNI_OnLoad function should look something like this if
written in C++:

To instead use "discovery" of native methods, you need to name them in a specific way (see
the JNI spec
for details). This means that if a method signature is wrong, you won't know about it until the
first time the method is actually invoked.

If you have only one class with native methods, it makes sense for the call to
System.loadLibrary to be in that class. Otherwise you should probably make the call
from Application so you know that it's always loaded, and always loaded early.

Any FindClass calls made from JNI_OnLoad will resolve classes in the
context of the class loader that was used to load the shared library. Normally
FindClass uses the loader associated with the method at the top of the Java
stack, or if there isn't one (because the thread was just attached) it uses the "system" class
loader. This makes JNI_OnLoad a convenient place to look up and cache class object
references.

64-bit considerations

To support architectures that use 64-bit pointers, use a long field rather than an
int when storing a pointer to a native structure in a Java field.

Unsupported features/backwards compatibility

All JNI 1.6 features are supported, with the following exception:

DefineClass is not implemented. Android does not use
Java bytecodes or class files, so passing in binary class data
doesn't work.

For backward compatibility with older Android releases, you may need to
be aware of:

Dynamic lookup of native functions

Until Android 2.0 (Eclair), the '$' character was not properly
converted to "_00024" during searches for method names. Working
around this requires using explicit registration or moving the
native methods out of inner classes.

Detaching threads

Until Android 2.0 (Eclair), it was not possible to use a pthread_key_create
destructor function to avoid the "thread must be detached before
exit" check. (The runtime also uses a pthread key destructor function,
so it'd be a race to see which gets called first.)

Weak global references

Until Android 2.2 (Froyo), weak global references were not implemented.
Older versions will vigorously reject attempts to use them. You can use
the Android platform version constants to test for support.

Until Android 4.0 (Ice Cream Sandwich), weak global references could only
be passed to NewLocalRef, NewGlobalRef, and
DeleteWeakGlobalRef. (The spec strongly encourages
programmers to create hard references to weak globals before doing
anything with them, so this should not be at all limiting.)

From Android 4.0 (Ice Cream Sandwich) on, weak global references can be
used like any other JNI references.

Local references

Until Android 4.0 (Ice Cream Sandwich), local references were
actually direct pointers. Ice Cream Sandwich added the indirection
necessary to support better garbage collectors, but this means that lots
of JNI bugs are undetectable on older releases. See
JNI Local Reference Changes in ICS for more details.

In Android versions prior to Android 8.0, the
number of local references is capped at a version-specific limit. Beginning in Android 8.0,
Android supports unlimited local references.

Determining reference type with GetObjectRefType

Until Android 4.0 (Ice Cream Sandwich), as a consequence of the use of
direct pointers (see above), it was impossible to implement
GetObjectRefType correctly. Instead we used a heuristic
that looked through the weak globals table, the arguments, the locals
table, and the globals table in that order. The first time it found your
direct pointer, it would report that your reference was of the type it
happened to be examining. This meant, for example, that if
you called GetObjectRefType on a global jclass that happened
to be the same as the jclass passed as an implicit argument to your static
native method, you'd get JNILocalRefType rather than
JNIGlobalRefType.

FAQ: Why do I get UnsatisfiedLinkError?

When working on native code it's not uncommon to see a failure like this:

java.lang.UnsatisfiedLinkError: Library foo not found

In some cases it means what it says — the library wasn't found. In
other cases the library exists but couldn't be opened by dlopen(3), and
the details of the failure can be found in the exception's detail message.

Common reasons why you might encounter "library not found" exceptions:

The library doesn't exist or isn't accessible to the app. Use
adb shell ls -l <path> to check its presence
and permissions.

The library wasn't built with the NDK. This can result in
dependencies on functions or libraries that don't exist on the device.

The method isn't being found due to a name or signature mismatch. This
is commonly caused by:

For lazy method lookup, failing to declare C++ functions
with extern "C" and appropriate
visibility (JNIEXPORT). Note that prior to Ice Cream
Sandwich, the JNIEXPORT macro was incorrect, so using a new GCC with
an old jni.h won't work.
You can use arm-eabi-nm
to see the symbols as they appear in the library; if they look
mangled (something like _Z15Java_Foo_myfuncP7_JNIEnvP7_jclass
rather than Java_Foo_myfunc), or if the symbol type is
a lowercase 't' rather than an uppercase 'T', then you need to
adjust the declaration.

For explicit registration, minor errors when entering the
method signature. Make sure that what you're passing to the
registration call matches the signature in the log file.
Remember that 'B' is byte and 'Z' is boolean.
Class name components in signatures start with 'L', end with ';',
use '/' to separate package/class names, and use '$' to separate
inner-class names (Ljava/util/Map$Entry;, say).

Using javah to automatically generate JNI headers may help
avoid some problems.

FAQ: Why didn't FindClass find my class?

(Most of this advice applies equally well to failures to find methods
with GetMethodID or GetStaticMethodID, or fields
with GetFieldID or GetStaticFieldID.)

Make sure that the class name string has the correct format. JNI class
names start with the package name and are separated with slashes,
such as java/lang/String. If you're looking up an array class,
you need to start with the appropriate number of square brackets and
must also wrap the class with 'L' and ';', so a one-dimensional array of
String would be [Ljava/lang/String;.
If you're looking up an inner class, use '$' rather than '.'. In general,
using javap on the .class file is a good way to find out the
internal name of your class.

If the class name looks right, you could be running into a class loader
issue. FindClass wants to start the class search in the
class loader associated with your code. It examines the call stack,
which will look something like:

Foo.myfunc(Native Method)
Foo.main(Foo.java:10)

The topmost method is Foo.myfunc. FindClass
finds the ClassLoader object associated with the Foo
class and uses that.

This usually does what you want. You can get into trouble if you
create a thread yourself (perhaps by calling pthread_create
and then attaching it with AttachCurrentThread). Now there
are no stack frames from your application.
If you call FindClass from this thread, the
JavaVM will start in the "system" class loader instead of the one associated
with your application, so attempts to find app-specific classes will fail.

There are a few ways to work around this:

Do your FindClass lookups once, in
JNI_OnLoad, and cache the class references for later
use. Any FindClass calls made as part of executing
JNI_OnLoad will use the class loader associated with
the function that called System.loadLibrary (this is a
special rule, provided to make library initialization more convenient).
If your app code is loading the library, FindClass
will use the correct class loader.

Pass an instance of the class into the functions that need
it, by declaring your native method to take a Class argument and
then passing Foo.class in.

Cache a reference to the ClassLoader object somewhere
handy, and issue loadClass calls directly. This requires
some effort.

FAQ: How do I share raw data with native code?

You may find yourself in a situation where you need to access a large
buffer of raw data from both managed and native code. Common examples
include manipulation of bitmaps or sound samples. There are two
basic approaches.

You can store the data in a byte[]. This allows very fast
access from managed code. On the native side, however, you're
not guaranteed to be able to access the data without having to copy it. In
some implementations, GetByteArrayElements and
GetPrimitiveArrayCritical will return actual pointers to the
raw data in the managed heap, but in others it will allocate a buffer
on the native heap and copy the data over.

The alternative is to store the data in a direct byte buffer. These
can be created with java.nio.ByteBuffer.allocateDirect, or
the JNI NewDirectByteBuffer function. Unlike regular
byte buffers, the storage is not allocated on the managed heap, and can
always be accessed directly from native code (get the address
with GetDirectBufferAddress). Depending on how direct
byte buffer access is implemented, accessing the data from managed code
can be very slow.

The choice of which to use depends on two factors:

Will most of the data accesses happen from code written in Java
or in C/C++?

If the data is eventually being passed to a system API, what form
must it be in? (For example, if the data is eventually passed to a
function that takes a byte[], doing processing in a direct
ByteBuffer might be unwise.)

If there's no clear winner, use a direct byte buffer. Support for them
is built directly into JNI, and performance should improve in future releases.

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